When designing modern internal combustion engines, especially supercharged engines, the exhaust temperature is an important parameter to be taken into consideration, due to firstly the relatively large gas flow in supercharged engines relative to the cylinder volume, secondly the higher exhaust counter-pressure when using catalytic exhaust converters leading to elevated exhaust temperatures, and thirdly the tendency to knock resulting from the desired to keep a relatively high compression ratio for preserving good engine suction characteristics.
A known method of limiting the maximum level of the exhaust gas temperature is to inject a cooling medium into the combustion chamber of the engine at those engine load conditions where there is risk that the temperature may exceed the maximum permissible level. Water can be used as a cooling medium injected into the combustion chamber through special injectors. The most common method is however to use extra fuel as a cooling medium and to quite simply use the ordinary engine injection system for making the engine fuel-air mixture richer.
Known systems for limiting exhaust gas temperature by injecting a cooling agent lack feedback circuits, and this means that one must assume a worst case situation when calculating the amount to be injected. The system must be designed with the assumption that the engine is operated with low octane fuel at high ambient temperature with maximum load and that these conditions shall prevail for a long period of time. This results in poor fuel economy since when driving normally even with a high load there is seldom any need to add extra fuel and temporary increases in load, such as when passing, do not bring the exhaust temperature to a critical value. Another disadvantage is that the exhaust gas temperature will be unnecessarily low when driving on high octane fuel at normal ambient temperature, and this results in a non-optimal fuel-air mixture ratio.